Let's say I have an array x and a mask for the array mask. I want to use np.copyto to write to x using mask. Is there a way I can do this? Just trying to use copyto doesn't work, I suppose because the masked x is not writeable.
x = np.array([1,2,3,4])
mask = np.array([False,False,True,True])
np.copyto(x[mask],[30,40])
x
# array([1, 2, 3, 4])
# Should be array([1, 2, 30, 40])
As commented index assignment works
In [16]: x[mask]=[30,40]
In [17]: x
Out[17]: array([ 1, 2, 30, 40])
You have to careful when using x[mask]. That is 'advanced indexing', so it creates a copy, not a view of x. With direct assignment that isn't an issue, but with copyto x[mask] is passed as an argument to the function.
In [19]: y=x[mask]
In [21]: np.copyto(y,[2,3])
changes y, but not x.
Checking its docs I see the copyto does accept a where parameter, which could be used as
In [24]: np.copyto(x,[0,0,31,41],where=mask)
In [25]: x
Out[25]: array([ 1, 2, 31, 41])
x[mask] = [30,40]work?